Electronic navigation devices employing Global Positioning System (“GPS”) receivers are known. The GPS includes a plurality of satellites that are in orbit about the Earth. The orbit of each satellite is not necessarily synchronous with the orbits of other satellites and, in fact, is likely asynchronous. The GPS receiver device receives spread spectrum GPS satellite signals from the various satellites. The spread spectrum signals continuously transmitted from each satellite utilize a highly accurate frequency standard accomplished with an extremely accurate atomic clock. Each satellite, as part of its data signal transmission, transmits a data stream indicative of that particular satellite. The GPS receiver device acquires spread spectrum GPS satellite signals from at least three satellites to calculate its two-I dimensional position by triangulation. Acquisition of an additional signal, resulting in signals from a total of four satellites, permits the GPS receiver device to calculate its three-dimensional position. In this manner, an electronic navigation device employing a GPS receiver has the ability to accurately compute the position of the device in real time, even as the device moves. One example of an electronic navigation device is the eMAP portable electronic map manufactured by Garmin International.
Although GPS enabled devices are often used to describe navigation devices, it will be readily appreciated that satellites need not be used at all to determine a geographic position of a receiving unit, since cellular towers or any customized transmitting radio frequency towers can be deployed and combined in groups of three or more. With such a configuration, any standard geometric triangulation algorithm can be used to determine an approximate location of the receiving unit.
In recent years, attempts have been made to combine navigation and geographic positioning services on other types of multipurpose devices, e.g. PDAs, cell phones and other intelligent appliances/apparel of the like. PDAs, for example, are small, substantially hand-held computers that are used for storing, manipulating and retrieving data. One example of a PDA is the Palm Pilot® manufactured by 3Com Corporation.
Plug-in GPS receiver modules for PDAs are known. There are problems associated with various plug-in GPS receiver modules and PDAs. One problem is that a PDA with a plug-in GPS receiver module is cumbersome to handle and use. Two separate devices must be handled. Additionally, the plug-in GPS receiver module and the PDA do not function together to provide integrated PDA features.
Further, among portable electronic navigation devices, including PDAs with plug-in receiver modules, there does not exist a back-up mechanism for continuing navigation related services when positioning signal reception is lost. This is particularly problematic when such devices are used in “urban-canyons” such as street level in a city beneath towering sky scrapers which “shade” or block satellite reception. Likewise, such devices typically lose their navigation related services once they are taken indoors within buildings.
Therefore, there exists a need for a portable electronic device that incorporates triangulation positioning functionality with a complementary navigation related functionality such that the device can continue to provide navigation related services in “urban-canyons” or indoors. Further, there exists a need for a portable electronic device that integrates triangulation positioning functionality with other handheld device functionality, e.g. cell phone and/or PDA functionality, in a manner which is not cumbersome to handle or to use.